Using a search list extension from another extension

[michael.k...@gmx.at]

Assume weewx-extension a provides search list extension A,  weewx-extension b provides search list extension B, and I want to use A when running a report in B.

Both weewx extension come with their skin.conf:

[CheetahGenerator]
    search_list_extensions = user.a.A

and 

[CheetahGenerator]
    search_list_extensions = user.b.B

No problem, just configure in weewx.conf

[StdReport]
  [[B]]
    [[[CheetahGenerator]]]
      search_list_extensions = user.a.A, user.b.B

But what if weewx-extension b receives an update and provides search list extension C?

b's skin.conf changes to

[CheetahGenerator]
    search_list_extensions = user.b.B, user.b.C

But the specific weewx installations fails to run the report [[B]], because the search_list_extension configuration is overridden, and the error log is just providing an error message, that is far from telling an obvious reason?

Is there a possibility to merge the values for search_list_extensions? Or is the only way adding the new search list extension also in weewx.conf?

For the author of extension B, is there a way to check if required extension configs are present, when running a report and provide a meaningful error message, if not?